This baseplate compass from Suunto includes a novel little addition; a small tracking wheel for measuring distances on scale maps.

The Suunto A-40 is a baseplate type compass with rotating luminous bezel and markings. The compass itself has a fixed declination correction scale. Below the bezel is a lanyard with snap-lock. This enables the compass to be easily detached from the lanyard.

There are various features on the transparent acrylic baseplate. The compass has a 50mm rule with 1mm graduations on one side of the baseplate. There are circular and triangular holes so that the user can mark starting and end points and places of note on a map. There is also a magnifying lens for reading map symbols.

What is of most interest to me however, is the curvimeter included at the end of the compass baseplate. This has a rotating tracking wheel at one end and, when rolled forward on a map, is capable of measuring up to 60cm (600mm) in 1mm increments, or different distances, depending on the scale of the map. It is specifically designed and printed to be used with 1:15 000, 1:25 000 and 1:50 000 scale maps. 1:15 000 scale is most commonly used with orienteering maps. The last two scales are particularly suited for use with orange O.S. Explorer and pink O.S. Landranger sheets.

Suunto was a Finish company founded in 1936 by engineer Tuomas Vohlonen. He invented a new method of producing liquid-filled compasses. They added marine and orienteering compasses to their portfolio in the 1950s and a diving compass in 1965. Since 2019 the company has had Chinese ownership. Suunto compasses such as the A-40 are intended for non-military use and have red baseplate prints. While the map measuring feature is pretty much intuitive, Suunto included some basic instructions on their multi-language pdf user guide for this now discontinued product.

The curvimeter tracking wheel rolls easily and is accurate. However the printed scales and other writing on the baseplate of my example is wearing badly and some is almost illegible. There was an alternative printing of the baseplate, and this image clearly shows the information, common to all examples, included on the baseplate.

The A-40 measures 121mm x 57mm x 10.4mm (across depth of bezel and baseplate) or 11.93mm (across diameter of curvimeter tracking wheel). It weighs 36g without lanyard, and 38.8g with lanyard. The curvimeter cannot be detached from the baseplate.

I am unsure when this compass first appeared on the market. Certainly it was included in Suunto’s 2009 product catalogue. The example I have is model SS012104013 for use in the northern hemisphere. There was also model SS012104014 for use in the southern hemisphere. Identical curvimeters were common to both models. The A-40 is no longer available from Suunto and wasn’t marketed for long. This may have been due to poor sales or durability issues with the curvimeter. It is an uncommon survivor and there is little else like it. Compass manufacturer Silva has produced baseplate compasses with slide on/off map measuring scales. These will be next month’s featured instrument. Links to other map measures that have previously featured can be found here.